Bovine lenticular gamma-glutamylcysteine synthetase: reaction sequence.

The sequence of substrate addition and product release during the reaction catalyzed by gamma-glutamylcysteine synthetase was investigated with purified enzyme from bovine lens. Thermal inactivation and kinetic studies suggest that L-glutamate is the first substrate to bind to the enzyme. L-beta-Chloroalanine was used as the L-cysteine analogue. Utilizing substrate activation and product inhibition studies, the following reaction sequence was determined: L-glutamate binding. ATP binding, ADP release, L-beta-chloroalanine binding, followed by inorganic phosphate and then dipeptide release. The implications of this mechanism with regard to control of the enzyme in situ and its importance in glutathione synthesis are discussed.